Environmental and regulatory initiatives are requiring ever-lower levels of both sulfur and aromatics in distillate fuels. For example, proposed sulfur limits for distillate fuels to be marketed in the European Union for the year 2005 is 50 wppm or less. There are also regulations that will require lower levels of total aromatics in hydrocarbons and, more specifically, to lower levels of multiring aromatics found in distillate fuels and heavier hydrocarbon products. Further, the maximum allowable aromatics level for U.S. on-road diesel, CARB reference diesel, and Swedish Class I diesel are 35, 10 and 5 vol. %, respectively. Further, the CARB and Swedish Class I diesel fuel regulations allow no more than 1.4 and 0.02 vol. % polyaromatics, respectively. Consequently, much work is presently being done in the hydrotreating art because of these proposed regulations.
Hydrotreating, or in the case of sulfur removal, hydrodesulfurization, is well known in the art and typically requires treating the petroleum streams with hydrogen in the presence of a supported catalyst at hydrotreating conditions. The catalyst is usually comprised of a Group VI metal with one or more Group VIII metals as promoters on a refractory support, such as alumina. Hydrotreating catalysts that are particularly suitable for hydrodesulfurization, as well as hydrodenitrogenation, generally contain molybdenum or tungsten on alumina promoted with a metal such as cobalt, nickel, iron, or a combination thereof. Cobalt promoted molybdenum on alumina catalysts are most widely used when the limiting specifications are hydrodesulfurization. Nickel promoted molybdenum on alumina catalysts are the most widely used for hydrodenitrogenation, partial aromatic saturation, as well as hydrodesulfurization.
One approach to prepare improved hydrotreating catalysts involved a family of phases structurally related to hydrotalcites and derived from the parent ammonium nickel molybdate. Whereas X-ray diffraction analysis has shown that hydrotalcites are composed of layered phases with positively charged sheets and exchangeable anions located in the galleries between the sheets, the related ammonium nickel molybdate phase has molybdate anions in interlayer galleries bonded to nickel oxyhydroxide sheets. See, for example, Levin, D., Soled, S. L., and Ying, J. Y., “Crystal Structure of an Ammonium Nickel Molybdate prepared by Chemical Precipitation,” Inorganic Chemistry, Vol. 35, No. 14, p. 4191-4197 (1996). The preparation of such materials also has been reported by Teichner and Astier, Appl. Catal. 72, 321-29 (1991), Ann. Chim. Fr. 12, 337-43 (1987), and C. R. Acad. Sci. 304 (II), #11, 563-6 (1987) and Mazzocchia, Solid State Ionics, 63-65 (1993) 731-35.
Another relatively new class of hydrotreating catalysts is described in U.S. Pat. Nos. 6,156,695, 6,162,350 and 6,299,760, all of which are incorporated herein by reference. The catalysts described in these patents are bulk multi-metallic catalysts comprised of at least one Group VIII non-noble metal (X) and at least two Group VIB metals, wherein the ratio of Group VIB metal to Group VIII non-noble metal is from about 10:1 to about 1:10. These catalysts are prepared from a precursor having the formula:(X)b(MO)c(W)dOz wherein the molar ratio of b:(c+d) is 0.5/1 to 3/1, the molar ratio of c:d is >0.01/1, preferably >0.1/1 and z=[2b+6(c+d)]/2. The precursor has x-ray diffraction peaks at d=2.53 and 1.70 Angstroms. The precursor is sulfided to produce the corresponding activated catalyst.
While such catalysts have proven to be superior to hydrotreating catalyst before their time, there still remains a need in the art for ever-more reactive and effective catalysts for removing heteroatoms, such as nitrogen and sulfur from hydrocarbon streams.